Clarify exit-first, then front-to-back selection

2.2. Path selection and constraints

   We choose the path for each new circuit before we build it.  We choose the

   exit node first, followed by the other nodes in the circuit.  All paths

   we generate obey the following constraints:

   exit node first, followed by the other nodes in the circuit, front to

   back. (In other words, for a 3-hop circuit, we first pick hop 3,

   then hop 1, then hop 2.)  All paths we generate obey the following

   constraints:

     - We do not choose the same router twice for the same path.

     - We do not choose any router in the same family as another in the same